The invention relates to aqueous dispersions of nanoureas which have a defined charge density on the particle surface, to a process for preparing them and to their use in binder.
The preparation of aqueous dispersions of nanoscale urea particles is described in WO-A 2005/063873. In that case hydrophilic isocyanates are introduced into water in the presence of a catalyst, which initiates crosslinking within the dispersed particles through urea bonds. These particles are used therein as additives for contact adhesives based on polychloroprene.
However, the nanourea dispersions described in WO-A 2005/063873, have certain disadvantages in relation to their production and also in relation to the stability of the dispersions during storage. In the course of the production of the nanoparticles by the process disclosed in WO-A 2005/063873, elimination of carbon dioxide takes place, leading to severe foaming of the reaction mixture, and in turn, to the considerable detriment of the reaction regime. Moreover, the dispersions described are not storage-stable, as there is unwanted evolution of gas during storage.
A further, substantial problem of the dispersions described in WO-A 2005/063873 lies in the inadequate solids content. Solids contents above 30% by weight are virtually impossible to produce with the procedure disclosed in WO-A 2005/063873. The nanourea dispersions described likewise exhibit compatibility problems with anionically hydrophilicized paints or adhesive dispersions, which sharply restricts their use in aqueous applications.
U.S. Pat. No. 4,171,391 describes anionically and cationically hydrophilicized polyurea particles which are prepared by breakdown of diisocyanates to polymer chains in water. These particles, however, do not have any crosslinking, and therefore exhibit elastomeric behavior. Moreover, as a result of ionic hydrophilicization, a high positive or negative surface charge, and hence a correspondingly high zeta potential is generated. As a result, a wide-ranging compatibility with other dispersions is not achieved.
It was an object of the present invention, therefore, to provide storage-stable nanourea dispersions which have a solids content of >30% by weight and are compatible with anionically hydrophilicized binders. A further intention was to provide a new process which solved the above-described problems in the process.
It has now been found that dispersions which comprise nanourea particles and possess a defined zeta potential are compatible with dispersions of anionically hydrophilicized polymers and have a solids content of more than 30% by weight. Moreover, a new process has been found for preparing the dispersions of the invention, in which there is no crosslinking of existing nanourea particles with the further-added polyisocyanates. Nor is there any grafting of the hydrophilicized polyisocyanates onto existing particles, which would lead to large particles and hence to unstable dispersions.